Organic/inorganic composite porous film and electrochemical device prepared thereby

ABSTRACT

Disclosed is an organic/inorganic composite porous film comprising: (a) inorganic particles; and (b) a binder polymer coating layer formed partially or totally on surfaces of the inorganic particles, wherein the inorganic particles are interconnected among themselves and are fixed by the binder polymer, and interstitial volumes among the inorganic particles form a micropore structure. A method for manufacturing the same film and an electrochemical device including the same film are also disclosed. An electrochemical device comprising the organic/inorganic composite porous film shows improved safety and quality.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation Application of co-pending U.S.application Ser. No. 15/266,838 filed on Sep. 15, 2016, which is aContinuation of U.S. application Ser. No. 13/785,260 filed on Mar. 5,2013 (now U.S. Pat. No. 9,490,463 issued on Nov. 8, 2016), which is aContinuation of U.S. application Ser. No. 11/217,918 filed on Sep. 1,2005 (now U.S. Pat. No. 8,409,746 issued on Apr. 2, 2013), which claimspriority under 35 U.S.C. 119(a) to Patent Application Nos.10-2004-70095, 10-2004-70096, and 10-2005-9999 filed in the Republic ofKorea on Sep. 2, 2004, Sep. 2, 2004, and Feb. 3, 2005 respectively, allof which are hereby expressly incorporated by reference into the presentapplication.

TECHNICAL FIELD

The present invention relates to a novel organic/inorganic compositeporous film that can show excellent thermal safety and lithium ionconductivity and a high degree of swelling with electrolyte compared toconventional polyolefin-based separators, and an electrochemical devicecomprising the same, which ensures safety and has improved quality.

BACKGROUND ART

Recently, there is an increasing interest in energy storage technology.

Batteries have been widely used as energy sources in portable phones,camcorders, notebook computers, PCs and electric cars, resulting inintensive research and development into them. In this regard,electrochemical devices are subjects of great interest. Particularly,development of rechargeable secondary batteries is the focus ofattention.

Secondary batteries are chemical batteries capable of repeated chargeand discharge cycles by means of reversible interconversion betweenchemical energy and electric energy, and may be classified into Ni-MHsecondary batteries and lithium secondary batteries. Lithium secondarybatteries include lithium secondary metal batteries, lithium secondaryion batteries, lithium secondary polymer batteries, lithium secondaryion polymer batteries, etc.

Because lithium secondary batteries have drive voltage and energydensity higher than those of conventional batteries using aqueouselectrolytes (such as Ni-MH batteries), they are produced commerciallyby many production companies. However, most lithium

secondary batteries have different safety characteristics depending onseveral factors. Evaluation of and security in safety of batteries arevery important matters to be considered. Therefore, safety of batteriesis strictly restricted in terms of ignition and combustion in batteriesby safety standards.

Currently available lithium ion batteries and lithium ion polymerbatteries use polyolefin-based separators in order to prevent shortcircuit between a cathode and an anode. However, because suchpolyolefin-based separators have a melting point of 200° C. or less,they have a disadvantage in that they can be shrunk or molten to cause achange in volume when the temperature of a battery is increased byinternal and/or external factors. Therefore, there is a greatpossibility of short-circuit between a cathode and an anode caused byshrinking or melting of separators, resulting in accidents such asexplosion of a battery caused by emission of electric energy. As aresult, it is necessary to provide a separator that does not cause heatshrinking at high temperature.

To solve the above problems related with polyolefin-based separators,many attempts are made to develop an electrolyte using an inorganicmaterial serving as a substitute for a conventional separator. Suchelectrolytes may be broadly classified into two types. The first type isa solid composite electrolyte obtained by using inorganic particleshaving lithium ion conductivity alone or by using inorganic particleshaving lithium ion conductivity mixed with a polymer matrix. See,Japanese Laid-Open Patent No. 2003-022707, [“Solid State Ionics”-vol.158, n. 3, p. 275, (2003)], [“Journal of Power Sources”-vol. 112, n. 1,p. 209, (2002)], [“Electrochimica Acta”-vol. 48, n. 14, p. 2003,(2003)], etc. However, it is known that such composite electrolytes arenot advisable, because they have low ion conductivity compared to liquidelectrolytes and the interfacial resistance between the inorganicmaterials and the polymer is high while they are mixed.

The second type is an electrolyte obtained by mixing inorganic particleshaving lithium ion conductivity or not with a gel polymer electrolyteformed of a polymer and liquid electrolyte. In this case, inorganicmaterials are introduced in a relatively small amount compared to thepolymer and liquid electrolyte, and thus merely have a supplementaryfunction to assist in lithium ion conduction made by the liquidelectrolyte.

However, because electrolytes prepared as described above have no porestherein or, if any, have pores with a size of several angstroms and lowporosity, formed by introduction of an artificial plasticizer, theelectrolytes cannot serve sufficiently as separator, resulting indegradation in the battery quality.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a schematic view showing an organic/inorganic composite porousfilm according to the present invention;

FIG. 2 is a photograph taken by Scanning Electron Microscope (SEM)showing the organic/inorganic composite porous film (PVdF-HFP/BaTiO₃)according to Example 1;

FIG. 3 is a photograph taken by SEM showing a polyolefin-based separator(PP/PE/PP) used in Comparative Example 1;

FIG. 4 is a photograph taken by SEM showing a porous film manufacturedby using a plasticizer according to Comparative Example 4;

FIG. 5 is a photograph showing the organic/inorganic composite porousfilm (PVdF-HFP/BaTiO₃) according to Example 1 compared to a currentlyused PP/PE/PP separator and PE separator, after each of the samples ismaintained at 150° C. for 1 hour;

FIG. 6 is a picture showing the results of an overcharge test for thelithium secondary battery including a currently used PP/PE/PP separatoraccording to Comparative Example 1 and the battery including theorganic/inorganic composite porous film (PVdF-HFP/BaTiO₃) according toExample 1; and

FIG. 7 is a graph showing variations in ion conductivity depending onthe content of inorganic particles, in the organic/inorganic compositeporous film according to the present invention.

DISCLOSURE OF THE INVENTION

We have found that an organic/inorganic composite porous film, formed byusing (1) inorganic particles and (2) a binder polymer, improves poorthermal safety of a conventional polyolefin-based separator.Additionally, we have found that because the organic/inorganic compositeporous film has a micropore structure formed by the inorganic particlespresent in the film, it provides an increased volume of space into whicha liquid electrolyte infiltrates, resulting in improvements in lithiumion conductivity and degree of swelling with electrolyte. Therefore, theorganic/inorganic composite porous film can improve the quality andsafety of an electrochemical device using the same as separator.

Therefore, it is an object of the present invention to provide anorganic/inorganic composite porous film capable of improving the qualityand safety of an electrochemical device, a method for manufacturing thesame and an electrochemical device comprising the same.

According to an aspect of the present invention, there is provided anorganic/inorganic composite porous film, which comprises (a) inorganicparticles; and (b) a binder polymer coating layer formed partially ortotally on the surface of the inorganic particles, wherein the inorganicparticles are interconnected among themselves and are fixed by thebinder polymer, and interstitial volumes among the inorganic particlesform a micropore structure. There is also provided an electrochemicaldevice (preferably, a lithium secondary battery) comprising the same.

According to another aspect of the present invention, there is provideda method for manufacturing an organic/inorganic composite porous film,which includes the steps of: (a) dissolving a binder polymer into asolvent to form a polymer solution; (b) adding inorganic particles tothe polymer solution obtained from step (a) and mixing them; and (c)coating the mixture of inorganic particles with binder polymer obtainedfrom step (b) on a substrate, followed by drying, and then detaching thesubstrate.

Hereinafter, the present invention will be explained in more detail.

The present invention is characterized in that it provides a novelorganic/inorganic composite porous film, which serves sufficiently asseparator to prevent electrical contact between a cathode and an anodeof a battery and to pass ions therethrough and shows excellent thermalsafety, lithium ion conductivity and degree of swelling withelectrolyte.

The organic/inorganic composite porous film is obtained by usinginorganic particles and a binder polymer. The uniform and heat resistantmicropore structure formed by the interstitial volumes among theinorganic particles permits the organic/inorganic composite porous filmto be used as separator. Additionally, if a polymer capable of beinggelled when swelled with a liquid electrolyte is used as the binderpolymer component, the organic/inorganic composite porous film can servealso as electrolyte.

Particular characteristics of the organic/inorganic composite porousfilm are as follows.

(1) The organic/inorganic composite porous film according to the presentinvention shows improved thermal safety by virtue of the inorganicparticles present therein.

In other words, although conventional polyolefin-based separators causeheat shrinking at high temperature because they have a melting point of120-140° C., the organic/inorganic composite porous film comprising theinorganic particles and binder polymer does not cause heat shrinking dueto the heat resistance of the inorganic particles. Therefore, anelectrochemical device using the above organic/inorganic compositeporous film as separator causes no degradation in safety resulting froman internal short circuit between a cathode and an anode even underextreme conditions such as high temperature, overcharge, etc. As aresult, such electrochemical devices have excellent safetycharacteristics compared to conventional batteries.

(2) Conventional solid electrolytes formed by using inorganic particlesand a binder polymer have no pore structure or, if any, have anirregular pore structure having a pore size of several angstroms.Therefore, they cannot serve sufficiently as spacer, through whichlithium ions can pass, resulting in degradation in the quality of abattery. On the contrary, the organic/inorganic composite porous filmaccording to the present invention has uniform micropore structuresformed by the interstitial volumes among the inorganic particles asshown in FIGS. 1 and 2, and the micropore structures permit lithium ionsto move smoothly therethrough. Therefore, it is possible to introduce alarge amount of electrolyte through the micropore structures so that ahigh degree of swelling with electrolyte can be obtained, resulting inimprovement in the quality of a battery.

(3) It is possible to control the pore size and porosity of theorganic/inorganic composite porous film by varying the particle diameterof the inorganic particles and the mixing ratio of the inorganicparticles with the polymer. The micropore structure is subsequentlyfilled with a liquid electrolyte so that the interfacial resistancegenerating among the inorganic particles or between the inorganicparticles and the binder polymer can be reduced significantly.

(4) When the inorganic particles used in the organic/inorganic compositeporous film have a high dielectric constant and/or lithium ionconductivity, the inorganic particles can improve lithium ionconductivity as well as heat resistance, thereby contributing toimprovement of battery quality.

(5) When the binder polymer used in the organic/inorganic compositeporous film is one showing a high degree of swelling with electrolyte,the electrolyte injected after assemblage of a battery can infiltrateinto the polymer and the resultant polymer containing the electrolyteinfiltrated therein has a capability of conducting electrolyte ions.Therefore, the organic/inorganic composite porous film according to thepresent invention can improve the quality of an electrochemical devicecompared to conventional organic/inorganic composite electrolytes.Additionally, the organic/inorganic composite porous film providesadvantages in that wettablity with an electrolyte is improved comparedto conventional hydrophobic polyolefin-based separators, and use of apolar electrolyte for battery is permitted.

(6) Finally, if the binder polymer is one capable of being gelled whenswelled with electrolyte, the polymer reacts with the electrolyteinjected subsequently and is gelled, thereby forming a gel typeorganic/inorganic composite electrolyte. Such electrolytes are producedwith ease compared to conventional gel-type electrolytes and showexcellent ion conductivity and a high degree of swelling withelectrolyte, thereby contributing to improve the quality of a battery.

One component present in the organic/inorganic composite porous filmaccording to the present invention is inorganic particles currently usedin the art. The inorganic particles permit interstitial volumes to beformed among them, thereby serving to form micropores and to maintainthe physical shape as spacer. Additionally, because the inorganicparticles are characterized in that their physical properties are notchanged even at a high temperature of 200° C. or higher, theorganic/inorganic composite porous film using the inorganic particlescan have excellent heat resistance.

There is no particular limitation in selection of inorganic particles,as long as they are electrochemically stable. In other words, there isno particular limitation in inorganic particles that may be used in thepresent invention, as long as they are not subjected to oxidation and/orreduction at the range of drive voltages (for example, 0-5 V based onLi/Li⁺) of a battery, to which they are applied. Particularly, it ispreferable to use inorganic particles having ion conductivity as high aspossible, because such inorganic particles can improve ion conductivityand quality in an electrochemical device. Additionally, when inorganicparticles having a high density are used, they have a difficulty indispersion during a coating step and may increase the weight of abattery to be manufactured. Therefore, it is preferable to use inorganicparticles having a density as low as possible. Further, when inorganicparticles having a high dielectric constant are used, they cancontribute to increase the dissociation degree of an electrolyte salt ina liquid electrolyte, such as a lithium salt, thereby improving the ionconductivity of the electrolyte.

For these reasons, it is preferable to use inorganic particles having ahigh dielectric constant of 5 or more, preferably of 10 or more,inorganic particles having lithium conductivity or mixtures thereof.

Particular non-limiting examples of inorganic particles having adielectric constant of 5 or more include BaTiO₃, Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT), PB(Mg₃Nb_(2/3))O₃—PbTiO₃(PMN-PT), hafnia (HfO₂), SrTiO₃, SnO₂, CeO₂, MgO, NiO, CaO, ZnO, ZrO₂,Y₂O₃, Al₂O₃, TiO₂, SiC or mixtures thereof.

As used herein, “inorganic particles having lithium ion conductivity”are referred to as inorganic particles containing lithium elements andhaving a capability of conducting lithium ions without storing lithium.Inorganic particles having lithium ion conductivity can conduct and movelithium ions due to defects present in their structure, and thus canimprove lithium ion conductivity and contribute to improve batteryquality. Non-limiting examples of such inorganic particles havinglithium ion conductivity include: lithium phosphate (Li₃PO₄), lithiumtitanium phosphate (Li_(x)Ti_(y)(PO₄)₃, 0<x<2, 0<y<3), lithium aluminumtitanium phosphate (Li_(x)Al_(y)Ti_(z)(PO₄)₃, 0<x<2, 0<y<1, 0<z<3),(LiAlTiP)_(x)O_(y) type glass (0<x<4, 0<y<13) such as14Li₂O-9Al₂O₃-38TiO₂-39P₂O₅, lithium lanthanum titanate(Li_(x)La_(y)TiO₃, 0<x<2, 0<y<3), lithium germanium thiophosphate(Li_(x)Ge_(y)P_(z)S_(w), 0<x<4, 0<y<1, 0<z<1, 0<w<5), such asLi_(3.25)Ge_(0.25)P_(0.75)S₄, lithium nitrides (Li_(x)N_(y), 0<x<4,0<y<2) such as Li₃N, SiS₂ type glass (Li_(x)Si_(y)S_(z), 0<x<3, 0<y<2,0<z<4) such as Li₃PO₄—Li₂S—SiS₂, P₂S₅ type glass (Li_(x)P_(y)S_(z),0<x<3, 0<y<3, 0<z<7) such as LiI—Li₂S—P₂S₅, or mixtures thereof.

According to the present invention, inorganic particles having arelatively high dielectric constant are used instead of inorganicparticles having no reactivity or having relatively low dielectricconstant. Further, the present invention also provides a novel use ofinorganic particles as separators.

The above-described inorganic particles, that have never been used asseparators, for example Pb(Zr,Ti)O₃ (PZT),Pb_(1-x)La_(x)Zr_(1-y)Ti_(y)O₃ (PLZT), Pb(Mg₃Nb_(2/3))O₃—PbTiO₃(PMN-PT), hafnia (HfO₂), etc., have a high dielectric constant of 100 ormore. The inorganic particles also have piezoelectricity so that anelectric potential can be generated between both surfaces by the chargeformation, when they are drawn or compressed under the application of acertain pressure. Therefore, the inorganic particles can preventinternal short circuit between both electrodes, thereby contributing toimprove the safety of a battery. Additionally, when such inorganicparticles having a high dielectric constant are combined with inorganicparticles having lithium ion conductivity, synergic effects can beobtained.

The organic/inorganic composite porous film according to the presentinvention can form pores having a size of several micrometers bycontrolling the size of inorganic particles, content of inorganicparticles and the mixing ratio of inorganic particles and binderpolymer. It is also possible to control the pore size and porosity.

Although there is no particular limitation in size of inorganicparticles, inorganic particles preferably have a size of 0.001-10 μm forthe purpose of forming a film having a uniform thickness and providing asuitable porosity. When the size is less than 0.001 μm, inorganicparticles have poor dispersibility so that physical properties of theorganic/inorganic composite porous film cannot be controlled with ease.When the size is greater than 10 μm, the resultant organic/inorganiccomposite porous film has an increased thickness under the same solidcontent, resulting in degradation in mechanical properties. Furthermore,such excessively large pores may increase a possibility of internalshort circuit being generated during repeated charge/discharge cycles.

The inorganic particles are present in the mixture of the inorganicparticles with binder polymer forming the organic/inorganic compositeporous film, preferably in an amount of 50-99 wt %, more particularly inan amount of 60-95 wt % based on 100 wt % of the total weight of themixture. When the content of the inorganic particles is less than 50 wt%, the binder polymer is present in such a large amount as to decreasethe interstitial volume formed among the inorganic particles and thus todecrease the pore size and porosity, resulting in degradation in thequality of a battery. When the content of the inorganic particles isgreater than 99 wt %, the polymer content is too low to providesufficient adhesion among the inorganic particles, resulting indegradation in mechanical properties of a finally formedorganic/inorganic composite porous film.

Another component present in the organic/inorganic composite porous filmaccording to the present invention is a binder polymer currently used inthe art. The binder polymer preferably has a glass transitiontemperature (T_(g)) as low as possible, more preferably T_(g) of between−200° C. and 200° C. Binder polymers having a low Tg as described aboveare preferable, because they can improve mechanical properties such asflexibility and elasticity of a finally formed film. The polymer servesas binder that interconnects and stably fixes the inorganic particlesamong themselves, and thus prevents degradation in mechanical propertiesof a finally formed organic/inorganic composite porous film.

When the binder polymer has ion conductivity, it can further improve thequality of an electrochemical device. However, it is not essential touse a binder polymer having ion conductivity. Therefore, the binderpolymer preferably has a dielectric constant as high as possible.Because the dissociation degree of a salt in an electrolyte depends onthe dielectric constant of a solvent used in the electrolyte, thepolymer having a higher dielectric constant can increase thedissociation degree of a salt in the electrolyte used in the presentinvention. The dielectric constant of the polymer may range from 1.0 to100 (as measured at a frequency of 1 kHz), and is preferably 10 or more.

In addition to the above-described functions, the binder polymer used inthe present invention may be further characterized in that it is gelledwhen swelled with a liquid electrolyte, and thus shows a high degree ofswelling. Therefore, it is preferable to use a polymer having asolubility parameter of between 15 and 45 MPa^(1/2), more preferably ofbetween 15 and 25 MPa^(1/2), and between 30 and 45 MPa^(1/2). Therefore,hydrophilic polymers having a lot of polar groups are more preferablethan hydrophobic polymers such as polyolefins. When the binder polymerhas a solubility parameter of less than 15 Mpa^(1/2) or greater than 45Mpa^(1/2), it has difficulty in swelling with a conventional liquidelectrolyte for battery.

Non-limiting examples of the binder polymer that may be used in thepresent invention include polyvinylidenefluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate,polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl polyvinylalcohol, cyanoethylcellulose,cyanoethylsucrose, pullulan, carboxymethyl cellulose,acrylonitrile-styrene-butadiene copolymer, polyimide or mixturesthereof. Other materials may be used alone or in combination, as long asthey satisfy the above characteristics.

The organic/inorganic composite porous film may further compriseadditives other than the inorganic particles and binder polymer.

When the organic/inorganic composite porous film is manufactured byusing inorganic particles and a binder polymer, the film may be realizedby three types of embodiments, but is not limited thereto.

The first type is an organic/inorganic composite porous film formed byusing a mixture of inorganic particles and binder polymer with noadditional substrate. The second type is an organic/inorganic compositeporous film formed by coating the mixture on a porous substrate havingpores, wherein the film coated on the porous substrate includes anactive layer obtained by coating the mixture of inorganic particles andbinder polymer on the surface of the porous substrate or on a part ofthe pores in the substrate. The third type is an organic/inorganiccomposite porous film formed by coating the Mixture on a cathode and/oran anode. The third type is a monolithic electrode and film.

In the second embodiment of the organic/inorganic composite porous filmaccording to the present invention, there is no particular limitation inthe substrate coated with the Mixture of inorganic particles and binderpolymer, as long as it is a porous substrate having pores. However, itis preferable to use a heat resistant porous substrate having a meltingpoint of 200° C. or higher. Such heat resistant porous substrates canimprove the thermal safety of the organic/inorganic composite porousfilm under external and/or internal thermal impacts. Non-limitingexamples of the porous substrate having a melting point of 200° C. orhigher that may be used include polyethylene terephthalate, polybutyleneterephthalate, polyester, polyacetal, polyamide, polycarbonate,polyimide, polyetherether ketone, polyether sulfone, polyphenyleneoxide, polyphenylene sulfidro, polyethylene naphthalene or mixturesthereof. However, other heat resistant engineering plastics may be usedwith no particular limitation.

Although there is no particular limitation in thickness of the poroussubstrate, the porous substrate preferably has a thickness of between 1μm and 100 μm, more preferably of between 5 μm and 50 μm. When theporous substrate has a thickness of less than 1 μm, it is difficult tomaintain mechanical properties. When the porous substrate has athickness of greater than 100 μm, it may function as resistance layer.

Although there is no particular limitation in pore size and porosity ofthe porous substrate, the porous substrate preferably has a porosity ofbetween 5% and 95%. The pore size (diameter) preferably ranges from 0.01μm to 50 μM, more preferably from 0.1 μm to 20 μm. When the pore sizeand porosity are less than 0.01 μm and 5%, respectively, the poroussubstrate may function as resistance layer. When the pore size andporosity are greater than 50 μm and 95%, respectively, it is difficultto maintain mechanical properties.

The porous substrate may take the form of a membrane or fiber. When theporous substrate is fibrous, it may be a nonwoven web forming a porousweb (preferably, spunbond type web comprising long fibers or melt blowntype web).

A spunbond process is performed continuously through a series of stepsand provides long fibers formed by heating and melting, which isstretched, in turn, by hot air to form a web. A melt blown processperforms spinning of a polymer capable of forming fibers through aspinneret having several hundreds of small orifices, and thus providesthree-dimensional fibers having a spider-web structure resulting frominterconnection of microfibers having a diameter of 10 μm or less.

The organic/inorganic composite porous film that may be formed invarious types of embodiments according to the present invention ischaracterized in that the film comprises a micropore structure. First,the organic/inorganic composite porous film formed by using the mixtureof inorganic particles and polymer alone has a micropore structureformed by interstitial volumes among the inorganic particles serving assupport as well as spacer. Next, the organic/inorganic composite porousfilm formed by coating the mixture on a porous substrate has porestructures present both in the substrate and in the active layer due tothe pores present in the porous substrate itself and interstitialvolumes among the inorganic particles in the active layer formed on thesubstrate. Finally, the organic/inorganic composite porous film obtainedby coating the mixture on the surface of an electrode has a uniform porestructure formed by interstitial volumes among the inorganic particlesin the same manner as the pore structure formed by electrode activematerial particles in the electrode. Therefore, any embodiment of theorganic/inorganic composite porous film according to the presentinvention has an increased volume of space, into which an electrolyteinfiltrates, by virtue of such micropore structures. As a result, it ispossible to increase dispersibility and conductivity of lithium ions,resulting in improvement in the quality of a battery.

The pore size and porosity of the organic/inorganic composite porousfilm mainly depend on the size of inorganic particles. For example, wheninorganic particles having a particle diameter of 1 μm or less are used,pores formed thereby also have a size of 1 μm or less. The porestructure is filled with an electrolyte injected subsequently and theelectrolyte serves to conduct ions. Therefore, the size and porosity ofthe pores are important factors in controlling the ion conductivity ofthe organic/inorganic composite porous film. Preferably, the pores sizeand porosity of the organic/inorganic composite porous film according tothe present invention range from 0.01 to 10 μm and from 5 to 95%,respectively.

There is no particular limitation in thickness of the organic/inorganiccomposite porous film according to the present invention. The thicknessmay be controlled depending on the quality of a battery. According tothe present invention, the film preferably has a thickness of between 1and 100 μm, more preferably of between 2 and 30 μm. Control of thethickness of the film may contribute to improve the quality of abattery.

There is no particular limitation in mixing ratio of inorganic particlesto polymer in the organic/inorganic composite porous film according tothe present invention. The mixing ratio can be controlled according tothe thickness and structure of a film to be formed finally.

The organic/inorganic composite porous film may be applied to a batterytogether with a microporous separator (for example, a polyolefin-basedseparator), depending on the characteristics of a finally formedbattery.

The organic/inorganic composite porous film may be manufactured by aconventional process known to one skilled in the art. One embodiment ofa method for manufacturing the organic/inorganic composite porous filmaccording to the present invention, includes the steps of: (a)dissolving a binder polymer into a solvent to form a polymer solution;(b) adding inorganic particles to the polymer solution obtained fromstep (a) and mixing them; and (c) coating the mixture obtained from step(b) on the surface of a substrate, followed by drying, and thendetaching the substrate.

Hereinafter, the method for manufacturing the organic/inorganiccomposite porous film according to the present invention will beexplained in detail.

(1) First, a binder polymer is dissolved in a suitable organic solventto provide a polymer solution.

It is preferable that the solvent has a solubility parameter similar tothat of the binder polymer to be used and a low boiling point. Suchsolvents can be mixed uniformly with the polymer and can be removedeasily after coating the polymer. Non-limiting examples of the solventthat may be used include acetone, tetrahydrofuran, methylene chloride,chloroform, dimethylformamide, N-methyl-2-pyrrolidone (NMP),cyclohexane, water and mixtures thereof.

(2) Next, inorganic particles are added to and dispersed in the polymersolution obtained from the preceding step to provide a mixture ofinorganic particles with binder polymer.

It is preferable to perform a step of pulverizing inorganic particlesafter adding the inorganic particles to the binder polymer solution. Thetime needed for pulverization is suitably 1-20 hours. The particle sizeof the pulverized particles ranges preferably from 0.001 and 10 μm.Conventional pulverization methods, preferably a method using a ballmill may be used.

Although there is no particular limitation in composition of the mixturecontaining inorganic particles and binder polymer, such composition cancontribute to control the thickness, pore size and porosity of theorganic/inorganic composite porous film to be formed finally.

In other words, as the weight ratio (I/P) of the inorganic particles (I)to the polymer (P) increases, porosity of the organic/inorganiccomposite porous film according to the present invention increases.Therefore, the thickness of the organic/inorganic composite porous filmincreases under the same solid content (weight of the inorganicparticles weight of the binder polymer). Additionally, the pore sizeincreases in proportion to the pore formation among the inorganicparticles. When the size (particle diameter) of inorganic particlesincreases, interstitial distance among the inorganic particles alsoincreases, thereby increasing the pore size.

(3) The mixture of inorganic particles with binder polymer is coated ona substrate, followed by drying, and then the substrate is detached toprovide the organic/inorganic composite porous film.

Particular examples of the substrate that may be used include Teflonsheets or the like generally used in the art, but are not limitedthereto.

In order to coat the porous substrate with the mixture of inorganicparticles and binder polymer, any methods known to one skilled in theart may be used. It is possible to use various processes including dipcoating, die coating, roll coating, comma coating or combinationsthereof.

In this step, when the substrate is a porous substrate having pores or apreformed electrode, various types of organic/inorganic composite porousfilms can be obtained. The mixture of inorganic particles and polymermay be coated on the surface of porous substrate, on the surface ofelectrode, and on a part of the pores present in the substrate. In thisstep, the step of detaching a substrate may be omitted.

The organic/inorganic composite porous film according to the presentinvention, obtained as described above, may be used as separator in anelectrochemical device, preferably in a lithium secondary battery.Additionally, the organic/inorganic composite porous film may be coatedwith a conventional polymer (for example, a polymer capable of beingswelled with an electrolyte) on one surface or both surfaces so as to beused as separator.

If the binder polymer used in the film is a polymer capable of beinggelled when swelled with a liquid electrolyte, the polymer may reactwith the electrolyte injected after assembling a battery by using theseparator, and thus be gelled to form a gel type organic/inorganiccomposite electrolyte.

The gel type organic/inorganic composite electrolyte according to thepresent invention is prepared with ease compared to gel type polymerelectrolytes according to the prior art, and has a large space to befilled with a liquid electrolyte due to its microporous structure,thereby showing excellent ion conductivity and a high degree of swellingwith electrolyte, resulting in improvement in the quality of a battery.

Further, the present invention provides an electrochemical devicecomprising: (a) a cathode; (b) an anode; (c) the organic/inorganiccomposite porous film according to the present invention, interposedbetween the cathode and anode; and (d) an electrolyte.

Such electrochemical devices include any devices in whichelectrochemical reactions occur and particular examples thereof includeall kinds of primary batteries, secondary batteries, fuel cells, solarcells or capacitors. Particularly, the electrochemical device is alithium secondary battery including a lithium secondary metal battery,lithium secondary ion battery, lithium secondary polymer battery orlithium secondary ion polymer battery.

According to the present invention, the organic/inorganic compositeporous film contained in the electrochemical device serves as separator.If the polymer used in the film is a polymer capable of being gelledwhen swelled with electrolyte, the film may serve also as electrolyte.

In addition to the above organic/inorganic composite porous film, amicroporous separator may also be used. Particular examples of themicroporous separator that may be used includes currently usedpolyolefin-based separators or at least one porous substrate having amelting point of 200° C., selected from the group consisting ofpolyethylene terephthalate, polybutylene terephthalate, polyester,polyacetal, polyamide, polycarbonate, polyimide, polyetherether ketone,polyether sulfone, polyphenylene oxide, polyphenylene sulfidro andpolyethylene naphthalene.

The electrochemical device may be manufactured by a conventional methodknown to one skilled in the art. In one embodiment of the method formanufacturing the electrochemical device, the electrochemical device isassembled by using the organic/inorganic composite porous filminterposed between a cathode and an anode, and then an electrolyte isinjected.

The electrode that may be applied together with the organic/inorganiccomposite porous film according to the present invention may be formedby applying an electrode active material on a current collectoraccording to a method known to one skilled in the art. Particularly,cathode active materials may include any conventional cathode activematerials currently used in a cathode of a conventional electrochemicaldevice. Particular non-limiting examples of the cathode active materialinclude lithium intercalation materials such as lithium manganeseoxides, lithium cobalt oxides, lithium nickel oxides, lithium ironoxides or composite oxides thereof. Additionally, anode active materialsmay include any conventional anode active materials currently used in ananode of a conventional electrochemical device. Particular non-limitingexamples of the anode active material include lithium intercalationmaterials such as lithium metal, lithium alloys, carbon, petroleum coke,activated carbon; graphite or other carbonaceous materials. Non-limitingexamples of a cathode current collector include foil formed of aluminum,nickel or a combination thereof. Non-limiting examples of an anodecurrent collector include foil formed of copper, gold, nickel, copperalloys or a combination thereof.

The electrolyte that may be used in the present invention includes asalt represented by the formula of A⁺B⁻, wherein A⁺ represents an alkalimetal cation selected from the group consisting of Li⁺, Na⁺, K⁺ andcombinations thereof, and B⁻ represents an anion selected from the groupconsisting of PF₆ ⁻, BF₄ ⁻, Cl⁻, Br⁻, I⁻, ClO₄ ⁻, AsF₆ ⁻, CH₃CO₂ ⁻,CF₃SO₃ ⁻, N(CF₃SO₂)₂ ⁻, C(CF₂SO₂)₃ ⁻ and combinations thereof, the saltbeing dissolved or dissociated in an organic solvent selected from thegroup consisting of propylene carbonate (PC), ethylene carbonate (EC),diethyl carbonate (DEC), dimethyl carbonate (DMC), dipropyl carbonate(DPC), dimethyl sulfoxide, acetonitrile, dimethoxyethane,diethoxyethane, tetrahydrofuran, N-methyl-2-pyrrolidone (NMP),ethylmethyl carbonate (EMC), gamma-butyrolactone (GBL) and mixturesthereof. However, the electrolyte that may be used in the presentinvention is not limited to the above examples.

More particularly, the electrolyte may be injected in a suitable stepduring the manufacturing process of an electrochemical device, accordingto the manufacturing process and desired properties of a final product.In other words, electrolyte may be injected, before an electrochemicaldevice is assembled or in a final step during the assemblage of anelectrochemical device.

Processes that may be used for applying the organic/inorganic compositeporous film to a battery include not only a conventional winding processbut also a lamination (stacking) and folding process of a separator andelectrode.

When the organic/inorganic composite porous film according to thepresent invention is applied to a lamination process, it is possible tosignificantly improve the thermal safety of a battery, because a batteryformed by a lamination and folding process generally shows more severeheat shrinking of a separator compared to a battery formed by a windingprocess. Additionally, when a lamination process is used, there is anadvantage in that a battery can be assembled with ease by virtue ofexcellent adhesion of the polymer present in the organic/inorganiccomposite porous film according to the present invention. In this case,the adhesion can be controlled depending on the content of inorganicparticles and polymer, and properties of the polymer. More particularly,as the polarity of the polymer increases and as the glass transitiontemperature (T_(g)) or melting point (T_(m)) of the polymer decreases,higher adhesion between the organic/inorganic composite porous film andelectrode can be obtained.

BEST MODE FOR CARRYING OUT THE INVENTION

Reference will now be made in detail to the preferred embodiments of thepresent invention. It is to be understood that the following examplesare illustrative only and the present invention is not limited thereto.

Reference Example. Variations in Ion Conductivity Depending on Contentof Inorganic Particles

The organic/inorganic composite system according to the presentinvention was observed to determine variations in ion conductivitydepending on the content of inorganic particles.

The organic/inorganic composite film according to the present inventionwas dipped into the electrolyte formed of ethylene carbonate/propylenecarbonate/diethyl carbonate (EC/PC/DEC=30:20:50 on the basis of wt %)containing 1M lithium hexafluorophosphate (LiPF₆) dissolved therein. Thefilm, into which the electrolyte is impregnated, was measured for ionconductivity by using Metrohm 712 instrument at a temperature of 25° C.

As shown in FIG. 7, as the content of inorganic particles increases, ionconductivity also increases. Particularly, when 50 wt % or more ofinorganic particles are used, ion conductivity increases significantly.

Example 1-9 Preparation of Organic/Inorganic Composite Porous Film andManufacture of Lithium Secondary Battery Using the Same Example 1

1-1. Preparation of Organic/Inorganic Composite Porous Film(PVdF-HFP/BaTiO₃)

PVdF-HFP polymer (polyvinylidene fluoride-hexafluoropropylene copolymer)was added to tetrahydrofuran (THF) in the amount of about 5 wt % anddissolved therein at 50° C. for about 12 hours or more to form a polymersolution. To the polymer solution obtained as described above, BaTiO₃powder having a particle diameter of about 400 nm was added with theconcentration of 20 wt % on the total solid content basis, and thendispersed to form a mixed solution (BaTiO₃/PVIDF-HFP=80:20 (weightratio)). Then, the mixed solution obtained as described above was coatedon a Teflon sheet by using a doctor blade coating method. After coating,THF was dried and the Teflon sheet was detached to obtain a finalorganic/inorganic composite porous film (see, FIG. 1). The final filmhad a thickness of about 30 μm. After measuring with a porosimeter, thefinal organic/inorganic composite porous film had a pore size of 0.4 μmand a porosity of 60%.

1-2. Manufacture of Lithium Secondary Battery

(Manufacture of Cathode)

To N-methyl-2-pyrrolidone (NMP) as a solvent, 94 wt % of lithium cobaltcomposite oxide (LiCoO₂) as cathode active material, 3 wt % of carbonblack as conductive agent and 3 wt % of PVdF (polyvinylidene fluoride)as binder were added to form slurry for a cathode. The slurry was coatedon Al foil having a thickness of 20 μm as cathode collector and dried toform a cathode.

(Manufacture of Anode)

To N-methyl-2-pyrrolidone (NMP) as solvent, 96 wt % of carbon powder asanode active material, 3 wt % of PVdF (polyvinylidene fluoride) asbinder and 1 wt % of carbon black as conductive agent were added to formmixed slurry for an anode. The slurry was coated on Cu foil having athickness of 10 μm as anode collector and dried to form an anode.

(Manufacture of Battery)

The cathode and anode obtained as described above were stacked with theorganic/inorganic composite porous film obtained as described in Example1-1 to form an assembly. Then, an electrolyte (ethylene carbonate(EC)/propylene carbonate (PC)/diethyl carbonate (DEC)=30:20:50 (wt %)containing 1M of lithium hexafluorophosphate (LiPF₆)) was injectedthereto to provide a lithium secondary battery.

Example 2

Example 1 was repeated to provide a lithium secondary battery, exceptthat mixed powder of BaTiO₃ and Al₂O₃ (weight ratio=20:80) was usedinstead of BaTiO₃ powder to obtain an organic/inorganic composite porousfilm (PVdF-HFP/BaTiO₃—Al₂O₃). After measuring with a porosimeter, thefinal organic/inorganic composite porous film had a thickness of 25 μm,pore size of 0.3 μm and a porosity of 57%.

Example 3

Example 1 was repeated to provide a lithium secondary battery, exceptthat PMNPT (lead magnesium niobate-lead titanate) powder was usedinstead of BaTiO₃ powder to obtain an organic/inorganic composite porousfilm (PVdF-HFP/PMNPT). After measuring with a porosimeter, the finalorganic/inorganic composite porous film had a thickness of 30 μm, poresize of 0.3 μm and a porosity of 60%.

Example 4

Example 1 was repeated to provide a lithium secondary battery, exceptthat PVdF-HFP was not used but about 2 wt % of carboxymethyl cellulose(CMC) polymer was added to water and dissolved therein at 60° C. forabout 12 hours or more to form a polymer solution, and the polymersolution was used to obtain an organic/inorganic composite porous film(CMC/BaTiO₃). After measuring with a porosimeter, the finalorganic/inorganic composite porous film had a thickness of 25 μm, poresize of 0.4 μm and a porosity of 56%.

Example 5

Example 1 was repeated to provide a lithium secondary battery, exceptthat PZT powder was used instead of BaTiO₃ powder to obtain anorganic/inorganic composite porous film (PVdF-HFP/PZT). After measuringwith a porosimeter, the final organic/inorganic composite porous filmhad a thickness of 25 μm, pore size of 0.4 μm and a porosity of 62%.

Example 6

Example 1 was repeated to provide a lithium secondary battery, exceptthat PLZT powder was used instead of BaTiO₃ powder to obtain anorganic/inorganic composite porous film (PVdF-HFP/PLZT). After measuringwith a porosimeter, the final organic/inorganic composite porous filmhad a thickness of 25 μm, pore size of 0.3 μm and a porosity of 58%.

Example 7

Example 1 was repeated to provide a lithium secondary battery, exceptthat HfO₂ powder was used instead of BaTiO₃ powder to obtain anorganic/inorganic composite porous film (PVdF-HFP/HfO₂). After measuringwith a porosimeter, the final organic/inorganic composite porous filmhad a thickness of 28 μm, pore size of 0.4 μm and a porosity of 60%.

Example 8

Example 1 was repeated to provide a lithium secondary battery, exceptthat lithium titanium phosphate (LiTi₂(PO₄)₃) powder having a particlediameter of about 400 nm was used in an amount of the total solidcontent of 20 wt %, instead of BaTiO₃ powder, to obtain anorganic/inorganic composite porous film (PVdF-HFP/LiTi₂(PO₄)₃) having athickness of about 20 μm. After measuring with a porosimeter, the finalorganic/inorganic composite porous film had a pore size of 0.5 μm andporosity of 62%.

Example 9

Example 1 was repeated to provide a lithium secondary battery, exceptthat mixed powder of BaTiO₃ and LiTi₂(PO₄)₃ (weight ratio=50:50) wasused instead of BaTiO₃ powder to obtain an organic/inorganic compositeporous film (PVdF-HFP/LiTi₂(PO₄)₃—BaTiO₃). After measuring with aporosimeter, the final organic/inorganic composite porous film had athickness of 25 μm, pore size of 0.3 μm and a porosity of 60%.

Comparative Examples 1-4 Comparative Example 1

Example 1 was repeated to provide a lithium secondary battery, exceptthat a conventional poly propylene/polyethylene/polypropylene (PP/PE/PP)separator (see, FIG. 3) was used.

Comparative Example 2

Example 1 was repeated to provide an organic/inorganic composite porousfilm and lithium secondary battery comprising the same, except thatBaTiO₃ and PVDF-HFP were used in a weight ratio of 20:80. Aftermeasuring the BaTiO₃/PVdF-HFP with a porosimeter, the finalorganic/inorganic composite porous film had a pore size of 0.01 μm orless and a porosity of about 10%.

Comparative Example 3

Example 1 was repeated to provide an organic/inorganic composite porousfilm and lithium secondary battery comprising the same, except thatLiTi₂(PO₄)₃ and PVDF-HFP were used in a weight ratio of 10:90. Aftermeasuring the LiTi₂(PO₄)₃/PVdF-HFP with a porosimeter, the finalorganic/inorganic composite porous film had a pore size of 0.01 μm orless and a porosity of about 5%.

Comparative Example 4. Manufacture of Porous Film Using Plasticizer

Dimethyl carbonate (DMC) was selected as plasticizer and used along withPVdF-HFP in a ratio of 30:70 (on the wt % basis) and THF as solvent toform a porous film. Dimethyl carbonate used in the film as plasticizerwas extracted from the film by using methanol to provide a final porousfilm and a lithium secondary battery comprising the same. Aftermeasuring the porous PVdF-HFP film with a porosimeter, the porous filmhad a pore size of 0.01 μm or less and a porosity of about 30% (see,FIG. 4).

Experimental Example 1. Surface Analysis of Organic/Inorganic CompositePorous Film

The following test was performed to analyze the surface of anorganic/inorganic composite porous film according to the presentinvention.

The sample used in this test was PVdF-HFP/BaTiO₃ obtained according toExample 1. As controls, a PP/PE/PP separator according to ComparativeExample 1 and the porous film using a plasticizer according toComparative Example 4 were used.

When analyzed by using Scanning Electron Microscope (SEM), the PP/PE/PPseparator according to Comparative Example 1 and the porous filmaccording to Comparative Example 4 showed a conventional microporousstructure (see, FIGS. 3 and 4). More particularly, the porous filmaccording to Comparative Example 4 had a dense pore structure formedindependently from the inorganic particles present on the surface of thefilm. It is thought that the dense pore structure is formed byartificial extraction of the plasticizer.

On the contrary, the organic/inorganic composite porous film accordingto the present invention showed a micropore structure formed by theinorganic particles as main component of the film (for example,inorganic particles with a high dielectric constant and/or lithium ionconductivity). Additionally, it could be seen that the polymer wascoated on the surface of the inorganic particles (see, FIG. 2).

Experimental Example 2. Evaluation of Heat Shrinkage ofOrganic/Inorganic Composite Porous Film

The following experiment was performed to compare the organic/inorganiccomposite porous film with a conventional separator.

The organic/inorganic composite porous film (PVdF-CTFE/BaTiO₃) accordingto Example 1 was used as sample. A conventional PP/PE/PP separator andPE separator were used as controls.

Each of the test samples was checked for its heat shrinkage after storedat a high temperature of 150° C. for hour. The test samples provideddifferent results after the lapse of 1 hour at 150° C. The PP/PE/PPseparator as control was shrunk due to high temperature to leave onlythe outer shape thereof. Similarly, the PE separator was shrunk to about1/10 of its original size. On the contrary, the organic/inorganiccomposite porous film according to the present invention showed goodresults with no heat shrinkage (see, FIG. 5)

As can be seen from the foregoing, the organic/inorganic compositeporous film according to the present invention has excellent thermalsafety.

Experimental Example 3. Evaluation for Safety of Lithium SecondaryBattery

The following test was performed to evaluate the safety of each lithiumsecondary battery comprising the organic/inorganic composite porous filmaccording to the present invention.

Lithium secondary batteries according to Examples 1-9 were used assamples. As controls, used were the battery using a currently usedPP/PE/PP separator according to Comparative Example 1, the battery usingthe BaTiO₃/PVdF-HFP film (weight ratio=20:80 on the wt % basis) asseparator according to Comparative Example 2, and the battery using theLiTi₂(PO₄)₃/PVdF-HFP film (weight ratio=10:90 on the wt % basis) asseparator according to Comparative Example 3.

3-1. Hot Box Test

Each battery was stored at high temperatures of 150° C. and 160° C. for1 hour and then checked. The results are shown in the following Table 1.

After storing at high temperatures, the battery using a currently usedPP/PE/PP separator according to Comparative Example 1 caused explosionwhen stored at 160° C. for 1 hour. This indicates that polyolefin-basedseparators cause extreme heat shrinking, melting and breakage whenstored at high temperature, resulting in internal short circuit betweenboth electrodes (i.e., a cathode and an anode) of a battery. On thecontrary, lithium secondary batteries comprising an organic/inorganiccomposite porous film according to the present invention showed such asafe state as to prevent firing and burning even at a high temperatureof 160° C. (see, Table 1).

Therefore, it can be seen that the lithium secondary battery comprisingan organic/inorganic composite porous film according to the presentinvention has excellent thermal safety.

TABLE 1 Hot Box Test Conditions 150° C./1 hr 160° C./1 hr Ex. 1 ◯ ◯ Ex.2 ◯ ◯ Ex. 3 ◯ ◯ Ex. 4 ◯ ◯ Ex. 5 ◯ ◯ Ex. 6 ◯ ◯ Ex. 7 ◯ ◯ Ex. 8 ◯ ◯ Ex. 9◯ ◯ Comp. Ex. 1 ◯ X Comp. Ex. 2 ◯ ◯ Comp. Ex. 3 ◯ ◯

3-2. Overcharge Test

Each battery was charged under the conditions of 6V/1A and 10V/1A andthen checked. The results are shown in the following Table 2.

After checking, the battery using a currently used PP/PE/PP separatoraccording to Comparative Example 1 exploded (see, FIG. 6). Thisindicates that the polyolefin-based separator is shrunk by overcharge ofthe battery to cause short circuit between electrodes, resulting indegradation in safety of the battery. On the contrary, each lithiumsecondary battery comprising an organic/inorganic composite porous filmaccording to the present invention showed excellent safety underovercharge conditions (see, Table 2 and FIG. 6).

TABLE 2 Overcharge Test Conditions 6 V/1 A 10 V/1 A Ex. 1 ◯ ◯ Ex. 2 ◯ ◯Ex. 3 ◯ ◯ Ex. 4 ◯ ◯ Ex. 5 ◯ ◯ Ex. 6 ◯ ◯ Ex. 7 ◯ ◯ Ex. 8 ◯ ◯ Ex. 9 ◯ ◯Comp. Ex. 1 ◯ X Comp. Ex. 2 ◯ ◯ Comp. Ex. 3 ◯ ◯

Experimental Example 4. Evaluation for Quality of Lithium SecondaryBattery

The following tests were performed in order to determine thecharge/discharge capacity of each lithium secondary battery comprisingan organic/inorganic composite porous film according to the presentinvention.

Lithium secondary batteries according to Examples 1-9 were used assamples. As controls, used were the battery using a currently usedPP/PE/PP separator according to Comparative Example 1, the battery usingthe BaTiO₃/PVdF-HFP film (weight ratio=20:80 on the wt % basis) asseparator according to Comparative Example 2, the battery using theLiTi₂(PO₄)₃/PVdF-HFP film (weight ratio=10:90 on the wt % basis) asseparator according to Comparative Example 3, and the battery using theporous PVdF-HFP film obtained by using a plasticizer as separatoraccording to Comparative Example 4.

Each battery having a capacity of 760 mAh was subjected to cycling at adischarge rate of 0.5 C, 1 C and 2 C. The following Table 3 shows thedischarge capacity of each battery, the capacity being expressed on thebasis of C-rate characteristics.

After performing the test, the battery according to Comparative Examples2 using, as separator, an organic/inorganic composite porous film thatincludes a mixture containing inorganic particles with a high dielectricconstant and a binder polymer in a ratio of 20:80 (on the wt % basis)and the battery according to Comparative Examples 3 using, as separator,an organic/inorganic composite porous film that includes a mixturecontaining inorganic particles with lithium ion conductivity and abinder polymer in a ratio of 10:90 (on the wt % basis), showed asignificant drop in capacity depending on discharge rates, as comparedto the batteries using, as separators, the organic/inorganic compositeporous film obtained from the above Examples according to the presentinvention and a conventional polyolefin-based separator (see, Table 3).This indicates that such relatively low amount of inorganic particlescompared to the polymer may decrease the pore size and porosity in thepore structure formed by interstitial volume among the inorganicparticles, resulting in degradation in the quality of a battery.Additionally, the battery using the porous film having a pore structureartificially formed by using a plasticizer as separator according toComparative Example 4 also showed a significant drop in capacitydepending on discharge rates in the same manner as the batteriesaccording to Comparative Examples 2 and 3.

On the contrary, lithium secondary batteries comprising theorganic/inorganic composite porous film according to the presentinvention showed C-rate characteristics comparable to those of thebattery using a conventional polyolefin-based separator under adischarge rate of up to 2 C (see, Table 3).

TABLE 3 Discharge Rate (mAh) 0.5 C 1 C 2 C Ex. 1 757 746 694 Ex. 2 756748 693 Ex. 3 756 744 691 Ex. 4 758 747 694 Ex. 5 759 750 698 Ex. 6 755742 690 Ex. 7 758 747 694 Ex. 8 756 745 793 Ex. 9 757 746 792 Comp. Ex.1 758 746 693 Comp. Ex. 2 695 562 397 Comp. Ex. 3 642 555 385 Comp. Ex.4 698 585 426

INDUSTRIAL APPLICABILITY

As can be seen from the foregoing, the organic/inorganic compositeporous film according to the present invention comprises inorganicparticles and a binder polymer, wherein the inorganic particles areinterconnected among themselves and fixed by the binder polymer andinterstitial volumes among the inorganic particles form a heat resistantmicropore structure. Therefore, it is possible to increase the space tobe filled with an electrolyte, and thus to improve a degree of swellingwith electrolyte and lithium ion conductivity. As a result, theorganic/inorganic composite porous film according to the presentinvention contributes to improve the thermal safety and quality of alithium secondary battery using the same as separator.

While this invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not limited to thedisclosed embodiment and the drawings. On the contrary, it is intendedto cover various modifications and variations within the spirit andscope of the appended claims.

1. A lithium secondary battery comprising: a cathode; an anode; anelectrolyte; and a separator comprising: a porous substrate, which isnot a cathode and/or an anode, and an organic/inorganic composite porousfilm coated directly on a surface of the porous substrate, theorganic/inorganic composite porous film comprising: a binder polymerdisposed on the inorganic particles and interconnecting the inorganicparticles to each other; and interstitial volumes defined by theinterconnected inorganic particles, wherein said interstitial volumesdefined by the interconnected inorganic particles directly form auniform pore structure 3-dimensionally throughout the organic/inorganiccomposite porous film, wherein the organic/inorganic composite porousfilm is obtained by coating the mixture of the inorganic particles andbinder polymer on the surface of the porous substrate, wherein theorganic/inorganic composite porous film is not formed by coating themixture of the inorganic particles and binder polymer on the surface ofa cathode or an anode.
 2. The lithium secondary battery according toclaim 1, wherein the inorganic particles have a size of between 0.001 μmand 10 μm.
 3. The lithium secondary battery according to claim 1,wherein the inorganic particles are present in the mixture of inorganicparticles with the binder polymer in an amount of 50-99 wt % based on100 wt % of the mixture.
 4. The lithium secondary battery according toclaim 1, wherein the binder polymer has a glass transition temperature(Tg) of between −200° C. and 200° C.
 5. The lithium secondary batteryaccording to claim 1, wherein the binder polymer has a solubilityparameter of between 15 and 45 MPa^(1/2).
 6. The lithium secondarybattery according to claim 1, wherein the binder polymer is at least oneselected from the group consisting of polyvinylidienefluoride-co-hexafluoropropylene, polyvinylidenefluoride-co-trichloroethylene, polymethylmethacrylate,polyacrylonitrile, polyvinylpyrrolidone, polyvinyl acetate,polyethylene-co-vinyl acetate, polyethylene oxide, cellulose acetate,cellulose acetate butyrate, cellulose acetate propionate,cyanoethylpullulan, cyanoethyl, polyvinylalcohol, cyanoethylcellulose,cyanoethylsucrose pullulan, carboxymethyl cellulose,acrylonitrile-styrene-butadiene copolymer and polyimide.
 7. The lithiumsecondary battery according to claim 1, wherein the organic/inorganiccomposite porous film has a pore size of between 0.001 and 10 μm.
 8. Thelithium secondary battery according to claim 1, wherein theorganic/inorganic composite porous film has a porosity of between 5% and95%.
 9. The lithium secondary battery according to claim 1, wherein theorganic/inorganic composite porous film has a thickness of between 1 and100 μm.
 10. The lithium secondary battery according to claim 1, whichfurther comprises a microporous separator.
 11. The lithium secondarybattery according to claim 10, wherein the microporous separator is apolyolefin-based separator, or at least one porous substrate having amelting point of 200° C. or higher, selected from the group consistingof polyethylene terephthalate, polybutylene terephthalate, polyester,polyacetal, polyamide, polycarbonate, polyimide, polyetherether ketone,polyether sulfone, polyphenylene oxide, polyphenylene sulfidro andpolyethylene naphthalene.
 12. The lithium secondary battery of claim 1,wherein the composite porous film is obtained by directly coating theporous substrate with a slurry of dispersed inorganic particles in asolution obtained by dissolving a binder polymer in a solvent.
 13. Thelithium secondary battery of claim 1, wherein the composite porous filmis formed by applying a slurry of the inorganic particles and the binderpolymer directly on the surface of the porous substrate and then dryingthe coating.
 14. The lithium secondary battery of claim 1, wherein theinterstitial volumes have a substantially uniform size.
 15. A method formanufacturing the lithium secondary battery according to claim 1, whichcomprises the steps of: (a) dissolving a binder polymer into a solventto form a polymer solution; (b) adding inorganic particles to thepolymer solution obtained from step (a) and mixing them; and (c) coatingthe mixture of inorganic particles with binder polymer obtained fromstep (b) on a substrate, followed by drying, and then detaching thesubstrate.